Our invention relates generally to the establishment of compatibility between devices designed for data transfer with magnetic storage media, such as flexible magnetic disks commonly known as floppy disks, with different track densities (i.e. the number of data tracks on the disk per unit distance in its radial direction). More specifically, our invention concerns a method of rewriting part or whole of data that has been written on a floppy disk by a first floppy disk drive (FDD) with a first track density, by means of another FDD designed for data transfer with a second track density that is twice as high as the first track density. Our invention also specifically pertains to the improved construction of apparatus comprising the second FDD. Typically, the first track density is 48 tracks per inch (TPI) or 18.9 tracks per centimeter, and the second track density is 96 TPI or 37.8 tracks per centimeter.
While 5.25-inch FDDs were initially developed for data transfer with a track density of 48 TPI, more recently they have been adapted for a twice as high track density of 96 TPI, the primary purpose being greater data storage capacity. The 96-TPI track density has been chosen by reason of the desired compatibility of the 48- and 96-TPI FDDs with the same floppy disks. Both types of FDDs incorporate magnetic read/write head assemblies of the tunnel erase type for trimming track edges. Reference may be had to "Latest Floppy Disk Drives and Their Application Knowhow" by Shoji Takahashi, published June 10, 1984, for further details on the compatibility of the 48- and 96-TPI FDDs.
As an illustration of the compatibility of the 48- and 96-TPI FDDs, let us assume that the 48-TPI FDD writes on a floppy disk. Part or all of the data thus written on the disk can then be rewritten by the 96-TPI FDD that has been adapted for the 48-TPI capability. Then the rewritten disk may be read by the 48-TPI FDD, although it may of course be read by the 96-TPI FDD. Such compatibility between the 48- and 96-TPI FDDs stands on the requirement that there be little or no mistracking of the head assemblies of both FDDs. If this requirement is fulfilled, the old data that has been written by the 48-TPI FDD will be practically completely erased by the tunnel erase head assembly from both sides of each new data track during the rewriting of the disk by the 96-TPI FDD in the 48-TPI mode.
Actually, however, substantial mistracking will take place in either the 48- or 96-TPI FDD, due for example to unavoidable manufacturing errors. Such mistracking will result in the misalignment of the data tracks formedby the two different FDDs, to such an extent that the pair of tunnel erase heads of the 96-TPI FDD become unable to erase parts of the old data tracks created by the 48-TPI FDD. The incomplete erasure of the old data will present no problem at all if the new data is read by the 96-TPI FDD. A problem does, however, arise when the rewritten disk is read by the 48-TPI FDD, because then its head assembly will scan not only the new tracks but also the incompletely erased old tracks.